Noninvasive blood pressure (BP) measurement is the technology that measures the BP indirectly by the arterial vessel wall beat or arterial volume. There are two types of noninvasive BP measurement technology: intermissive measurement and continuous measurement. Intermissive measurement can get the value of BP at different time points. But due to the consistent change of BP between every two heart beats and every two time points, the systolic pressure and diastolic may not represent the meaningful value, of course, this two values relate to different heart beats. Continuous measurement technology, which measures the BP without intermission, can provide the beat-to-beat BP or continuous BP oscillation. It is very important to realize the noninvasive continuous BP measurement. But, until now, there is not an accurate method that can achieve the aim.
BP measurement in according to the pulse wave transit velocity (PWTV) is a type of noninvasive continuous BP measurement method. In 1922, Bazzett discovered that arterial pressure related to the PWTV or pulse wave transit time (PWTT), in addition to arterial volume and arterial flexibility. In 1957, Lansdown pointed that PWTT and arterial BP present linear relationship to some extent and this relationship is stable for a given subject in a period of time. Moreover, the coefficients that descript linear relationship between PWTT and BP vary violently for different subjects with different arterial vessel tissue structure. But in past studies, the BP of different subject was general evaluated through the same coefficient, so the result can be distorted by errors.
An equation describing the relationship between the BP and PWTT for a given subject can be deduced in terms of the linear relationship between them:BP=a+b×PWTT  (A)In this formula, b is regressing coefficient to be estimated which varies in different subjects. But for a given subject and in a short period, a and b are stable. Previous analyze shows that to evaluate arterial BP, the coefficient a and b for a given individual must be obtained firstly. After that, BP can be computed By the PWTT (or PWTV). The coefficient a and b need to be rectified by means of individualized regressing technology, so the BP computed by the regressing equation (A) can fit individual condition well.
In principle, evaluating two parameters needs two group independent experiment data. PWTT and mean arterial pressure in the quiet condition can be obtained, so coefficient a, i.e. intercept, is easy to get. To b=ΔBP/ΔPWTT, i.e. slope, is always estimated by altering BP to get two groups of data. But, in order to change BP, exercise or drugs were often involved in the experiment, which can change the artery character and violate the premise that in a short period linear relationship in equation (A) is consistent.
Yu Mengsun also believed that when body posture changed (for example, supine and elevating leg), PWTT in the elevating leg would change. It is because that the change of body posture alters the pressure in some vessel and then makes PWTV different from that of normal state. If experiment data in normal status and posture changing status can be obtained, the coefficient b will be estimated from these data. This method can rectify parameters more accurately, but multi-group information in relate to the beat-to-beat BP cannot be got continuously when the body posture changed.